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Supporting Online Materials for “Inducing Chalcogenide Phase Change with Ultra-Narrow
Carbon Nanotube Heaters” by F. Xiong, A. Liao, E. Pop, Univ. Illinois Urbana-Champaign
1. Methods of CNT Fabrication: Carbon nanotubes (CNTs) were grown by chemical vapor
deposition using a mixture of CH4 and C2H4 as the carbon feedstock, and H2 as the carrier gas at
900 oC. The flow rate of CH4 to C2H4 was kept large to grow predominantly single-walled CNTs.
Fe (~2 Å thick deposited by e-beam evaporation) is used as the catalyst for CNT growth. The
catalyst was deposited on ~100 nm thick SiO2 and highly p-doped Si wafers. Patterned catalyst
islands are formed using photolithography and lift-off. Prior to growth, catalysts were annealed
at 900 oC in Ar environment to ensure the formation of Fe nanoparticles, from which the CNTs
grow. The nanotubes were contacted with Ti/Pd (1/40 nm) electrodes defined using photolithography. The electrode separation on our test chips is varied from L ~ 1-10 μm.
2. GST Thin Film Deposition: GST thin film deposition is done in high vacuum using ATC
2000 custom four gun co-sputtering system (AJA International), with a deposition rate at 0.4 Å/s.
Deposition at this rate ensures that there is only minimal damage to the CNT from the sputtering
process. The sputtering target Ge2Sb2Te5 was purchased from ACI Alloys Incorporate. Thin film
thickness is characterized with X-ray reflectivity measurement using Philips Xpert Pro XRD system on control samples. By probing diffraction intensities at glancing angles of incidence, we are
able to confirm the GST thin film thickness is 10.0 nm ± 0.4 nm (Fig. S1a).
We have also performed atomic force microscopy (AFM) measurement on our samples before and after GST deposition (Fig. S1b and S1c, respectively). The measured surface roughness
of our devices only increased minimally from 0.3 nm to 0.5 nm.
A
Critical
angle
B
1st fringe
angle
2nd fringe
angle
C
FIG. S1. (A) X-ray reflectivity measurement of the sputtered GST thin film using control samples.
The control sample is fabricated by sputtering GST directly onto highly doped Si substrate
immediately after the removal of the native oxide layer by wet etching. The measurement
confirms the GST thin film thickness is 10 nm. (B) and (C) AFM images of a CNT before and
after GST deposition indicating the depositted GST thin film is conformal.
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3. Finite Element Modeling Parameters: The electrical resistivity and thermal conductivity of
GST are parameterized as a function of their crystalline state and the temperature using known
experimental results, as shown in Fig. S2.
B
A
FIG. S2. (A) Temperature dependence of GST electrical resistivity σGST(T).1 (B) Temperature
dependence of GST thermal conductivity kGST(T).2
The table below summarizes the thermal and electrical properties used for the Finite Element (FE)
simulation in this paper:
k (W/m/K) C (J/m3/K)
σ (S/m)
Pd
22
2.93 x 106
1 x 107
SiO2
1.4
1.72 x 106
1 x 10-16
GST
kGST(T)
1.24 x 106
σGST(T)
CNT
3000
1.10 x 106
σCNT(T)
REFERENCES
1
2
M. Lankhorst, B. Ketelaars, and R. Wolters, Nature Materials 4, 347 (2005).
H. K. Lyeo, D. G. Cahill, B. S. Lee, J. R. Abelson, M. H. Kwon, K. B. Kim, S. G. Bishop,
and B. K. Cheong, Applied Physics Letters 89, 151904 (2006).